The disclosure of Japanese Patent Application No. 2000-237463 filed on Aug. 4, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a vehicular braking control apparatus and method that switches from an automatic braking mode based on an inter-vehicle distance control or the like, to a manual braking mode of applying braking force in accordance with the amount of operation performed by an operating person.
2. Description of Related Art
A braking control apparatus is known which automatically controls the braking force during an automatic braking mode so as to maintain at least a predetermined distance from a vehicle running ahead. If an operating person performs a braking operation during execution of the automatic braking mode, a process of discontinuing the automatic braking mode and giving higher priority to the braking operation of the operating person is executed. For example, Japanese Patent Application Laid-Open No. 11-157424 discloses a technique for reducing a change in braking force that can occur at the time of a switch from the automatic braking mode to the manual braking mode.
In the technique described in Japanese Patent Application Laid-Open No. 11-157424, a predetermined delay time is set when a braking operation is performed by an operating person. The delay time is set in accordance with the brake pedal depressing speed, the vehicle speed, etc. For example, if the brake pedal depressing speed is great, the delay time is set to a reduced time. A master cylinder pressure after elapse of the delay time is estimated. Then, a control of reducing the wheel cylinder pressure is performed in accordance with a pressure reducing characteristic such that after the delay time actually elapses, the wheel cylinder pressure becomes equal to the estimated master cylinder pressure. In this manner, the change in braking force that can occur at the time of switching of the braking mode is reduced.
However, if the above-described control technique is adopted, it becomes necessary to perform various estimating processes, such as the setting of a delay time, the estimation of a master cylinder pressure, etc. Thus, the load of the estimating processes becomes great. Furthermore, if an error occurs in an estimated value obtained as described above, vibrations caused by a hydraulic pressure change at the time of switching of the brake mode propagate to the brake pedal in some cases. In such cases, an operating person will have an uncomfortable feeling. Furthermore, if such a control technique is adopted, it becomes necessary to provide a sensor for detecting the wheel cylinder pressure that is used for the estimation processes.
The invention has been accomplished to address the aforementioned problems. It is one object of the invention to provide a vehicle braking control apparatus and method capable of reliably reducing a sharp hydraulic pressure change at the time of switching of the braking mode while employing a relatively simple construction that does not need either a complicated logic for estimation or an extra detection sensor.
A vehicular baking control apparatus and method in accordance with the invention applies braking force to a vehicle and includes a braking system that applies a braking force to a wheel in accordance with a liquid pressure of an operating liquid, a liquid pressure actuator which has a liquid pressure source that raises the liquid pressure of the operating liquid, and which causes a predetermined liquid pressure to act on the braking system separately from a braking operation of a braking operation member that generates a signal to generate the liquid pressure based on a braking operation of an operating person, and a controller that controls operation of the liquid pressure actuator. The controller (a) controls the liquid pressure actuator so that the liquid pressure supplied to the braking system is reduced with a first gradient of reduction, when the operating person operates the braking operation member to a predetermined position, and (b) controls the liquid pressure actuator so that the liquid pressure supplied to the braking system is reduced with a second gradient of reduction that is different from the first gradient of reduction, when a state of the braking operation of the operating person is greater than a predetermined threshold.
At a stage where the braking operation member is operated to the predetermined position, it is determined that a relatively gentle brake operation that can achieve a slight deceleration has been performed. Then, the controller reduces the liquid pressure supplied to the braking system with the first gradient of reduction, for example, a relatively gentle gradient of reduction. If the state of braking operation of the operating person is greater than the predetermined threshold, it is determined that the amount of operation of the braking operation member is great and that the operation person has performed a significant brake operation. Then, the controller reduces the liquid pressure supplied to the braking system with a gradient of reduction that is different from the first gradient of reduction, for example, a relatively great gradient of reduction.
Thus, the invention controls the liquid pressure actuator so as to achieve a predetermined gradient of reduction in a case where the supply of the liquid pressure by the liquid pressure actuator ends, and is followed by a shift to the liquid pressure based on the braking operation of the braking operation member. Therefore, the liquid pressure can be gradually changed, so that vibrations that are caused by a change in the liquid pressure and that can propagate to the braking operation member can be reduced. Furthermore, if an operating person has performed a firm and significant brake operation, the liquid pressure under control can be quickly changed to the liquid pressure based on the braking operation performed by the operating person.